The present invention concerns a system and method for recording audio and video information onto a hard disk drive and in particular to apparatus and a method for concealing soft errors when the audio and video data are read from the hard disk.
Recently, several video recording products have been introduced in the consumer market that allow television viewers to record their favorite programs to a hard disk drive instead of a video tape. These systems include the ReplayTV system available from Replay Networks and the TiVo Personal TV Service available from TiVo, Inc. These systems allow viewers to record between 14 and 30 hours of television programs that may then be viewed in any order. In addition, these players allow a live video broadcast to be recorded and paused such that a viewer may begin watching the program in real time and, after the pause, continue watching the recorded program. One such system is described in U.S. Pat. No. 5,999,691 to Takagi et al. entitled TELEVISION RECEIVER, RECORDING AND REPRODUCTION DEVICE, DATA RECORDING METHOD AND DATA REPRODUCING METHOD.
These systems operate by receiving analog or compressed digital television signals, compressing the analog signals using, for example, the encoding standards specified by the Moving Picture Experts Group (MPEG), and storing the compressed digital video and audio data onto a hard disk drive.
The disk drives used in these systems tend to be more expensive than comparable disk drives used, for example, in conventional personal computer systems. This is because the digital television recorders do not tolerate disk soft errors. Disk soft errors are errors that occur the first time data is read but do not occur when the same data is read again. These errors can occur, for example, due to misalignment between the disk head and the data track, or due to a temporary change in the magnetic properties of the disk head in the disk drive.
In a conventional computer disk drive, these errors are handled in the disk controller. When the controller reads a data segment, it determines whether the data have been read correctly by applying a checksum, cyclic redundancy check (CRC) or error correction code to the retrieved data. If this process indicates errors in the data, the controller retries the read operation. That is to say, it causes the disk drive to reread the track from which the erroneous segment was taken. In many cases, the second or third time the data is read from the disk, no error occurs. In these instances, the original error is classified as a soft error. If the retry operation can not recover the data, the error is classified as a hard error.
In some disk drive systems, the interval between reading the data the first time and the second time includes an intermediate write step in which the disk head is positioned on an unused portion of the disk or on a portion of the disk containing known data and is activated to write a small amount of data. This operation counteracts a temporary change in the magnetic properties of the disk head which may occur during prolonged data read operations. Most disk drives include unused portions that serve as spare tracks. These tracks are substituted for bad tracks in the main part of the disk. Because the data written during the write operation performed after a soft error is not important, the operation may be performed on one of the spare segments or even on a known bad segment of the disk. Alternatively, all disk drives include track headers that contain known information. Rather than writing random information to a spare or bad track, the disk controller may re-write known data in a track header.
While the disk drive can usually fully recover data from a soft error, it is by definition unable to recover data from a hard error. Typically, disk drives handle errors of this type by marking the segment in which the hard error occurs as a bad disk segment. The segment is then not used for subsequent read and write operations and any data in that segment is lost.
The problem that video disk recorders have with disk soft errors relates to the disk latency between the first and second read operations. If, for example, the disk is operating at 3600 revolutions per minute (RPM), the rotational latency between the first disk read operation and the retry read operation consumes approximately 16 milliseconds. This is the same amount of time as is used to display one video field of an interlace scan video signal, or one video frame of a progressive scan video signal. Thus, if the disk drive performs one or more retry operations when a soft error occurs, the input data stream is effectively delayed by at least one field or frame interval. Data that is compressed according to the MPEG standard does not tolerate even slight delays in the input data stream. In the MPEG standard, each image has defined times at which it is to be decoded and displayed. If a soft error delays the recovery of data from the disk, the decoder may not be able to process the data in time to meet these timing requirements.
Conventional disk based video recording systems solve this problem by ignoring disk soft errors. When a disk error occurs whether it is a hard error or a soft error, the corrupted data is passed on to the decoder and the system does not retry the disk read operation. When the decoder recognizes that the data is corrupted, it may at least partially conceal the error using conventional drop out compensation circuits that, for example, repeat a previous field or frame to compensate for errors in the video stream and mute the audio signal to compensate for errors in the audio data stream. If the decoder does not recognize the data as being corrupted, it attempts to decode it, possibly causing additional audio and video distortion.
In addition, to compensate for the lack of disk soft error compensation, the disk drives used in conventional television recording systems may be designed to minimize soft errors. This undesirably adds to the cost of the disk based video system. The frequency of soft errors in a disk drive tends to increase as the drive ages. Consequently, the quality of the reproduced television programs decreases.